1. Field of the Invention
The present invention relates to a chromate free chemical solution for treating metal surfaces. More particularly, the present invention relates to an aqueous dipping solution for adding corrosion resistance to a metal surface and for providing the metal surface with a decorative color.
2. Prior Art
There are a variety of methods for treating metal surfaces for an equally wide variety of desired purposes. For example, many metal surfaces are treated to provide corrosion protection, abrasion resistance or to impart a desired decorative or identification color. Such methods include chromate conversion coating, immersion or electrolytic plating, metal coloring and electroless (autocatalytic) plating, etc. Although these methods do work for their intended purposes, there are certain problems associated with each method.
For example, in the chromate conversion coating method, the presence of hexavalent and trivalent chromium creates toxicity and pollution hazards. The problem with the chromate is that its use in treating metal surfaces results in a hazardous waste product. At present, using chromate requires a costly and time consuming effluent treatment process. Eventually it may be impossible to treat such waste products without investing in a complete reclamation system which is expected to be very expensive. As a result, there is presently a great need for a metal treating solution which will give the metal substrate corrosion resistance as well as the surface color provided by chromate treatment, without the pollution problems attendant with the use of chromate solutions.
Another disadvantage associated with the chromate conversion coating method is that chromate conversion coating is in general limited to treating aluminum, cadmium, copper, magnesium, silver and zinc, and is limited in the variety and depth of colors obtainable. These two problems are also associated with immersion plating. Further, the hazardous waste problem of the chromate conversion coating method is also present in processes for coloring metals with mordant dyes since such processes often require the formation of a chromate film on the surface of the metal substrate prior to coloring.
The closest method associated with the process of the present invention is known as staining or coloring of metals. Such processes are disclosed in German Pat. No. 149,566, German Pat. No. 163,067; Beutel, Bewahrte Arbeitsweisen der Metalfarbung, Second Edition, 1925; Groschuff, Phys. Techn. Reichs Inst. Deutche Mechaniker Zeitung, 1913, Vol. 22, pages 233-239, and U.S. Pat. No. 2,295,842. However, the staining or coloring method requires high heat, and typically, other post-treatment processes to remove oxides or other undesired precipitates from the metal substrate.
Therefore, it is an object of the present invention to provide a solution for treating metal surfaces which will impart the metal substrate with increased corrosion resistance and the metal substrate surface with a wide variety of decorative colors without producing hexavalent or trivalent chromium waste products. It is a further object of the present invention to provide a simple and inexpensive process for treating metal substrates to increase corrosion resistance and to impart a wide range of decorative colors with the process being performed at ambient temperature, primarily--and, or if desirable, temperature can be increased.